1. Technical Field of the Invention
The present invention relates in general to a Pulse Width Modulation (PWM) generator and particularly to a PWM generator providing improved duty cycle resolution.
2. Description of Related Art
Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with digital outputs. PWM outputs can be generated from a Microprocessor/Micro controller or a Programmable Logic Device. PWM is used in a wide variety of applications, ranging from measurement and communications to power control and conversion.
PWM is a way of digitally encoding analog signal levels. Through the use of high-resolution counters, the duty cycle of a square wave is modulated to encode a specific analog signal level. The PWM signal is still digital because, at any given instant of time, the full DC supply is either fully on or fully off. The voltage or current source is supplied to the analog load by means of a repeating series of on and off pulses. The on-time is the time during which the DC supply is applied to the load, and the off-time is the period during which that supply is switched off. With sufficient bandwidth as input, any analog value can be encoded with PWM.
The instant frequency of the analog input wave is proportional to the parameters as the instant value in the high-resolution counters, the frequency of the PWM waveform and the number of samples of the input analog waveform. PWM duty cycle can be controlled by improving the resolution of the PWM wave. The resolution of the PWM wave is proportional to the value in the high-resolution counter.
An existing system for closed loop control of duty cycle is disclosed in the patent European Patent No. 1087516, the disclosure of which is hereby incorporated by reference. The patent provides a system and a method for controlling the energization of an electric motor by producing a pulse width modulated command signal for controlling switching devices that connect the motor to an energization source. PWM duty cycle value is formed as a function of the PWM command signal. Further, a leading current with respect to the phase angle of the back EMF is provided to the motor as a function comparing the PWM duty cycle value with a particular value of the threshold voltage. Herein, the PWM duty cycle value is calculated as a function of the PWM command signal. The stated method does not provide precision measurement and control of the PWM duty cycle resolution, and is ineffective in providing an optimized control over PWM duty cycle.
U.S. Pat. No. 6,775,158 (see, FIG. 1) describes another conventional PWM controller, wherein the PWM controller is used to drive an inverter (800), and comprises a feedback circuit (400), a dead time circuit (500) for masking out synchronization conditions occurring when the dead time duration between switching the polarities of desired output AC waveform is greater than or equal to the duration of the PWM source waveform. It further includes a carrier range regulation circuit (210) for reducing PWM duty cycle loss. The PWM controller provides an application specific PWM controller and method. However, the controller does not provide maximal resolution for the PWM duty cycle. Further, the hardware used in the controller adds to the system cost and bulk of the circuitry.
Thus, a need is felt for an application specific method for improving PWM duty cycle resolution to a high precision factor that can be incorporated in a programmable logic device or an embedded system.